Revert "Remove revertRemovalOfSiblingPrerendering killswitch (#26549)"

This reverts commit 7b0642b.

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -38,6 +38,7 @@ import {
   enableCache,
   enableTransitionTracing,
   useModernStrictMode,
+  revertRemovalOfSiblingPrerendering,
   disableLegacyContext,
   alwaysThrottleRetries,
 } from 'shared/ReactFeatureFlags';
@@ -2544,14 +2545,28 @@ function completeUnitOfWork(unitOfWork: Fiber): void {
   // sibling. If there are no more siblings, return to the parent fiber.
   let completedWork: Fiber = unitOfWork;
   do {
-    if (__DEV__) {
+    if (revertRemovalOfSiblingPrerendering) {
       if ((completedWork.flags & Incomplete) !== NoFlags) {
-        // NOTE: If we re-enable sibling prerendering in some cases, this branch
-        // is where we would switch to the unwinding path.
-        console.error(
-          'Internal React error: Expected this fiber to be complete, but ' +
-            "it isn't. It should have been unwound. This is a bug in React.",
-        );
+        // This fiber did not complete, because one of its children did not
+        // complete. Switch to unwinding the stack instead of completing it.
+        //
+        // The reason "unwind" and "complete" is interleaved is because when
+        // something suspends, we continue rendering the siblings even though
+        // they will be replaced by a fallback.
+        // TODO: Disable sibling prerendering, then remove this branch.
+        unwindUnitOfWork(completedWork);
+        return;
+      }
+    } else {
+      if (__DEV__) {
+        if ((completedWork.flags & Incomplete) !== NoFlags) {
+          // NOTE: If we re-enable sibling prerendering in some cases, this branch
+          // is where we would switch to the unwinding path.
+          console.error(
+            'Internal React error: Expected this fiber to be complete, but ' +
+              "it isn't. It should have been unwound. This is a bug in React.",
+          );
+        }
       }
     }
 
@@ -2655,9 +2670,24 @@ function unwindUnitOfWork(unitOfWork: Fiber): void {
       returnFiber.deletions = null;
     }
 
-    // NOTE: If we re-enable sibling prerendering in some cases, here we
-    // would switch to the normal completion path: check if a sibling
-    // exists, and if so, begin work on it.
+    if (revertRemovalOfSiblingPrerendering) {
+      // If there are siblings, work on them now even though they're going to be
+      // replaced by a fallback. We're "prerendering" them. Historically our
+      // rationale for this behavior has been to initiate any lazy data requests
+      // in the siblings, and also to warm up the CPU cache.
+      // TODO: Don't prerender siblings. With `use`, we suspend the work loop
+      // until the data has resolved, anyway.
+      const siblingFiber = incompleteWork.sibling;
+      if (siblingFiber !== null) {
+        // This branch will return us to the normal work loop.
+        workInProgress = siblingFiber;
+        return;
+      }
+    } else {
+      // NOTE: If we re-enable sibling prerendering in some cases, this branch
+      // is where we would switch to the normal completion path: check if a
+      // sibling exists, and if so, begin work on it.
+    }
 
     // Otherwise, return to the parent
     // $FlowFixMe[incompatible-type] we bail out when we get a null

packages/shared/ReactFeatureFlags.js

@@ -22,6 +22,11 @@ export const enableComponentStackLocations = true;
 // when it rolls out to prod. We should remove these as soon as possible.
 // -----------------------------------------------------------------------------
 
+// This is phrased as a negative so that if someone forgets to add a GK, the
+// default is to enable the feature. It should only be overridden if there's
+// a regression in prod.
+export const revertRemovalOfSiblingPrerendering = false;
+
 // -----------------------------------------------------------------------------
 // Land or remove (moderate effort)
 //

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -54,6 +54,7 @@ export const enableScopeAPI = false;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = false;
 export const disableLegacyContext = false;
+export const revertRemovalOfSiblingPrerendering = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const enableSuspenseAvoidThisFallback = false;

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -36,6 +36,7 @@ export const enableScopeAPI = false;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = false;
 export const disableLegacyContext = false;
+export const revertRemovalOfSiblingPrerendering = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -36,6 +36,7 @@ export const enableScopeAPI = false;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = false;
 export const disableLegacyContext = false;
+export const revertRemovalOfSiblingPrerendering = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -36,6 +36,7 @@ export const enableScopeAPI = false;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = false;
 export const disableLegacyContext = false;
+export const revertRemovalOfSiblingPrerendering = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -36,6 +36,7 @@ export const enableScopeAPI = true;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = true;
 export const disableLegacyContext = false;
+export const revertRemovalOfSiblingPrerendering = false;
 export const enableTrustedTypesIntegration = false;
 export const disableTextareaChildren = false;
 export const disableModulePatternComponents = true;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -47,6 +47,12 @@ export const enableSchedulingProfiler = __VARIANT__;
 // so we don't need to use __VARIANT__ to get extra coverage.
 export const replayFailedUnitOfWorkWithInvokeGuardedCallback = __DEV__;
 
+// This flag only exists so it can be connected to a www GK that acts as a
+// killswitch. We don't run our tests against the `true` value because 1) it
+// affects too many tests 2) it shouldn't break anything. But it is mildly
+// risky, hence this extra precaution.
+export const revertRemovalOfSiblingPrerendering = false;
+
 // TODO: These flags are hard-coded to the default values used in open source.
 // Update the tests so that they pass in either mode, then set these
 // to __VARIANT__.

packages/shared/forks/ReactFeatureFlags.www.js

@@ -18,6 +18,7 @@ export const {
   disableInputAttributeSyncing,
   disableIEWorkarounds,
   enableTrustedTypesIntegration,
+  revertRemovalOfSiblingPrerendering,
   replayFailedUnitOfWorkWithInvokeGuardedCallback,
   enableLegacyFBSupport,
   enableDebugTracing,